The invention relates generally to a dichroic optical element, and more particularly to dichroic optical elements for use in a projection type display apparatus in which a projected image is formed using light valves.
A conventional dichroic optical element such as a polarized beam splitter, dichroic mirror or other device such as disclosed in U.S. Pat. Nos. 4,464,018; 4,191,456; and 4,461,542 is characterized by its polarized and wavelength separation properties. These properties are chosen independent of one another.
A typical, well known projection type display apparatus is disclosed in S.I.D. '86 Digest No. Page 375 (1986). The apparatus separates white light into colored light which is modulated by light valves in accordance with the image to be produced. The modulated colored lights are then synthesized (i.e., combined with one another) to form a color image which is projected toward a screen for display.
Conventional dichroic optical elements passively divide (i.e., separate) light but are not used to actively change the separating properties. White light can be separated into its primary colors (e.g., red, green and blue) by a first dichroic optical element. The primary colors are then synthesized by a second dichroic optical element having the same wavelength separation properties as the first dichroic optical element. In other words, the primary colors of light pass through two optical elements which have the same wavelength separation properties. The light intensity of the synthesized image is far less than the intensity of the white light prior to separation by the first optical element.
This reduction in light intensity can be understood by initial reference to FIG. 10(a) which illustrates the wavelength selection properties of a conventional dichroic optical element. The blue (B), green (G) and red (R) wavelengths of light each have skirts 1001, 1002 and 1003 hereinafter referred to as wavelength transition regions, respectively. FIG. 10(b) illustrates the light intensity spectrum of the red (R), green (G) and blue (B) color lights following synthesis, that is, after the colored lights are recombined. The light intensity of the primary colors of light in FIG. 10(b) is reduced by about 50% compared to the light intensity of the primary colors of FIG. 10(a). In particular, a significant loss of light from use of two optical elements having the same wavelength characteristics for separation and synthesis results in an inefficient use of light flux. To ensure that the synthesized colored light is of a suitable intensity, a high powered light source providing the incident white light to the first optical element is required.
Accordingly, it is desirable to provide a dichroic optical element which has different wavelength selection properties. It is also desirable to provide a projection type display device using two dichroic optical elements which more efficiently utilizes and thereby reduces the power requirements of the light source.